Molecular sieve desoption with a mixture of hydrocarbons



Nov. 10, 1970 R. `W. STOKELD, JR

'MOLECULAR SIEVE DESORPTION WITH A MIXTURE OF HYDROCARBONS Filed Oct.31, 1968 'United States Patent O1 hcc 3,539,501 Patented Nov. 10, 19703,539,501 MGLECULAR SIEVE DESORPTHON Wl'llI-l A MIXTURE F HYDRUCARBNSRichard W. Stokeld, Jr., Wappingers Falls, N.Y., assigner to TexacoInc., New York, N.Y., a corporation of Delaware Filed Oct. 31, 1968,Ser. No. 772,279 Int. Cl. C07c 7/12 U.S. Cl. 208-310 9 Claims ABSTRACTOF THE DISCLSURE A vapor phase process for the separation of straightchain hydrocarbon components from petroleum fractions by a molecularsieve selective adsorbent, comprising adsorption, purge and desorptionsteps, the purge and desorption steps being carried out using a straightchain hydrocarbon mixture comprising a major amount of at least oneheavy straight chain hydrocarbon having a molecular weight of from l to3 carbon atoms less than the molecular weight of the lightest straightchain hydrocarbon component of the petroleum fraction being separated bythe process and a minor amount of at least one light straight chainhydrocarbon having a molecular Weight of from to 7 carbon atoms lessthan the lightest straight chain hydrocarbon component of the petroleumfraction being separated by the process.

The present invention is related to an improved vapor phase method ofseparating straight chain hydrocarbons from petroleum fractions.

It is known from commonly assigned U.S. Pat. No. 3,373,103 that C10-C20straight chain hydrocarbons can be separated from vapor phase mixturesthereof with nonstraight chain hydrocarbons by a processing cycleincluding the steps of adsorption, purge and desorption carried out atelevated temperatures and pressures.

The improved process of the present invention is particularly directedto the separation and recovery of high molecular weight straight chainhydrocarbon from petroleum fractions containing mixtures of straightchain and non-straight chain hydrocarbons by a process comprising anadsorption step carried out at an elevated temperature andsuperatmospheric pressure wherein the vaporized petroleum fraction iscontacted in an adsorption zone containing a bed of selective adsorbentof Type 5 A. structure to adsorb the straight chain hydrocarboncomponents of the mixture therefrom in the pores of said adsorbent andto saturate same with the adsorbed straight chain hydrocarbons; theadsorption step is terminated when the adsorption zone contains astraight chain hydrocarbon overcharge of between 0.5 and by weight: adepressuring step wherein the pressure in the adsorption zone is reducedto a value below that employed in the adsorption step, but not belowatmospheric pressure; a purge step wherein the laden adsorbent iscontacted with a purge medium comprising a straight chain hydrocarbonmixture hereinafter more fully defined, in vapor phase to removesurface-adsorbed hydrocarbons and hydrocarbons in the void spaces of thebed therefrom; the purge step is discontinued; a repressuring step toattain a pressure in the adsorption vessel greater than the pressureduring the adsorption step; repressurization is terminated; a desorptionstep wherein the pore adsorbed straight chain hydrocarbons of theselective adsorbent are desorbed in the vapor phase with a desorbingmedium comprising a straight chain hydrocarbon mixture, as hereinaftermore fully dened; the desorption step is terminated when from 70 to 90%of the pore adsorbed components have been removed, a depressuring stepto reduce the pressure to the adsorption pressure; and repeating saidoperation 1n sequence.

The process of the present invention is particularly adaptable for theprocessing of high boiling petroleum fractions in the heavy/kerosine andgas oil ranges, i.e.` having boiling point temperatures ranging fromabout 325 to 670 F. and containing C10-C20 straight chain hydrocarbonsin relatively significant amounts on the order of 15% or more by weightand especially C13-C20 straight chain hydrocarbons.

The expression surfaceadsorbed hydrocarbons as used hereinabove includesall adsorption on the sieve other than in the sieve cages (within thezeolite crystal). The expression includes all the non-normal compoundsadsorbed in the macro-pores of the sieve (inter-crystalline pores) aswell as those adsorbed on the surface.

The expression straight chain hydrocarbon mixture as used in thespecification and claims in connection with the purge medium and thedesorbing medium refers to a mixture of straight chain hydrocarbonscontaining a major amount of at least one heavy straight chainhydrocarbon having a molecular weight of from 1 to 3 carbon atoms lessthan the lightest straight chain hydrocarbon component of the petroleumfraction being separated and a minor amount of at least one lightstraight chain hydrocarbon having a molecular weight of from 5 to 7carbon atoms less than the lightest straight chain hydrocarbon componentof the petroleum fraction being separated by the process.

The method of the present invention is particularly adaptable for theproduction of high purity normal parafns in excellent yields in a rapid,eiiicient and economical manner.

Accordingly, it is an object of the present invention to provide animproved hydrocarbon treating process. A further object is to provide animproved method of producing relatively high molecular weight straightchain hydrocarbons of a high degree of purity in commercially attractiveyields from mixtures of such hydrocarbon and non-straight chainhydrocarbons. A straight further object is to provide an improved cyclichydrocarbon separation process which is conducted in a relatively shortperiod of time.

How these and other objects of this invention are acomplished willbecome apparent with reference to the acompanying disclosure. In atleast one embodiment of the invention at least one of the foregoingobjects will be achieved.

By straight chain hydrocarbon is meant any aliphatic or acryclic or openchain hydrocarbon which does not possess side chain branching.Representative straight chain hydrocarbons are the normal parains andthe normal olens, mono- 0r polyolens, including the straight chainacetylenic hydrocarbons. The nonstraight chain hydrocarbons comprise thearomatic and naphthenic hydrocarbons as well as the isoparaffinic,isooletinic hydrocarbons and the like.

The practice of this invention is applicable to any solid selectiveadsorbent which selectively adsorbs straight chain hydrocarbons to thesubstantial exclusion of nonstraight chain hydrocarbons. This invention,however, is particularly applicable to a molecular sieve selectiveadsorbent such as a calcium aluminosilicate selective adsorbent,apparently actually a sodium calcium aluminosilicate, marketed by LindeCompany, and designated Linde Molecular Sieve Type 5A 0r 5A-45. Thecrystals of this particular calcium aluminosilicate have a pore size oropening of about 5 A. units, a pore size suiciently large to admitstraight chain hydrocarbons, such as the normal paraflins and the normalolens, to the substantial exclusion of the non-straight chainhydrocarbons, i.e.,

naphthenic, aromatic, isoparaffinic and isoolenfinic hydrocarbonsQThisparticular selective adsorbent is available in various sizes such as inthe form of 1/a" or 1/16" diameter extrusions, or as a finely dividedpowder having a particle size in the range of 0.5-5.0 microns. Ingeneral, a selective adsorbent employed in the practice of thisinvention may be in any suitable form or shape, granular, spheroidal ormicrospheroidal.

The method of the present invention should be carried out in the vaporphase and under essentially isothermal conditions. The particularoperating conditions selected are dependent on the nature of the feedstream to the adsorption zone, the carbon number range of the feedstream and desired product stream as well as the carbon numberdistribution (relative amounts of each carbon number) within the range,the straight chain hydrocarbon content of the feed stream and theolenic, sulfur, nitrogen and aromatic content thereof. In general, thefeed stream preferably should be relatively low in olens, sulfur,nitrogen and aromatics content and these impurities can be readilyreduced to acceptable limits or removed in a manner well known in theart such as by mild hydrogenation involving mild catalytic reforming. Inaddition, the feed stream should be relatively free from the lowermolecular weight hydrocarbons such as in the range from about C1 to C9as such light hydrocarbons complicate recovery of the desorbing medium.

In the accompanying drawing the single ligure thereof illustrates aschematic flow diagram of one method of carrying out the presentinvention. In the drawing, vessels 14, 14A and 14B represent the samesieve case in three different phases of the operating cycle, adsorption,purge and desorption phases.

In the drawing a vapor phase mixture of relatively high molecular Weightstraight chain and non-straight chain hydrocarbons is charged by way oflines 10 and 12 into a lower end of an adsorption vessel 14 maintainedat an elevated temperature and superatmospheric pressure containingtherein a bed of synthetic calcium sodium aluminosilicate of Type Astructure such as a Linde 5A- 45 Moleular Sieve. In the adsorptionvessel 14 the straight chain components of the feed mixture are adsorbedby the selective adsorbent. From the outlet end of the vessel 14 throughline 16 there is recovered a treated effluent mixture now containing asubstantially reduced amount of straight chain hydrocarbons therein aswell as desorbing medium present in the sieve cages from a previousdesorption step and the effluent is passed to a fractionator 18 fromwhich is recovered a non-straight chain hydrocarbon product stream byway of line 21 which can be passed to a storage vessel not shown for useas a fuel source and a desorbent recycle stream by way of line 19 whichcan be returned to desorption feed line 40 by way of line 23. Thedesorbent medium present in the adsorption etlluent is obtained from theprevious cycle wherein during desorption, a portion of the desorbingmedium is adsorbed by the sieve pores from which the higher molecularweight straight chain components have been removed.

At the completion of the adsorption step, hereinafter more fullydescribed, the feed in line to adsorption vessel 14 is discontinued. Inthe depressuring step, the vessel 14A is depressured by venting throughlines 26, 27, 28 and accumulator 54- maintained at about atmosphericpressure. When vessel 14A is at the selected loW pressure in thepressuring step, the purge step is begun. In the purging step, a streamof desorbing medium contained in line 40, 42 and 44 is introducedcountercurrent to the flow of the feed stream 10 into vessel 14A and apurge effluent stream is withdrawn therefrom by way of lines 26, 27 and28 and passed to purge accumulator 54. At the end of the purge step therepressuring step is commenced.

In the repressuring step, the flow of the stream of desorbing mediuminto vessel 14A by way of line 44 Was 4 continued to increase thepressure in the vessel to the selected desorbing pressure. When theselected desorbingr pressure is attained in vessel 14A the desorptionstep is begun.

In the desorption step, the desorbing medium in the vapor state ispassed through lines 40, 42 and 43 into the adsorbent vessel 14Bcontaining the straight chain hydrocarbon components adsorbed by theselective adsorbent. The flow of the desorbing medium is alsocountercurrent to the feed ow during the adsorption step.

Countercurrent desorption flow (i.e. opposite to the flow in the vesselduring adsorption) is highly desirable to assist in removing adsorbedstraight chain hydrocarbons from the selective adsorbent.

The resulting desorption effluent is withdrawn from vessel 14B by way ofline 36 and passed through lines 38 and 39 to fractionator 60 whereinthe desorbate and desorbing medium are separately recovered. Theadsorbed straight chain hydrocarbons in the desorbate are recovered fromfractionator 60 by way of line 64. The desorbing medium is recovered byway of line 62 and can be returned to line 40 for further use.

At the termination of the desorption step, vessel 14B is depressured toattain the lower pressure used in the adsorption step and the cycle isrepeated by introducing an additional quantity of fresh feed into vessel14 by Way of line 10 and line 12.

The adsorption step in the process of the present invention is carriedout with the feed stream being in the vapor phase.

The particular adsorption temperature used varies with the type ofcharge stock, carbon number content thereof, and desired range of thestraight chain hydrocarbons to be recovered from the charge stock.However, it is necessary to carry out the adsorption step at atemperature above the dew point of the vaporized feed stream to minimizesurface adsorption of the non-adsorbed hydrocarbons on the selectiveadsorbent and also to decrease the holdup of the charge stock in thesieve voids. A further requirement, which controls the upper temperaturelimit of the adsorption step is the need to avoid cracking of the chargestock. Keeping in mind these lower and upper temperature limitations, ithas been found that a temperature range of about S-675 F. in theadsorption step will permit excellent separations.

In the adsorption step, the adsorption vessel should be maintained at apositive pressure above atmospheric pressure to permit the selectiveadsorbent to adsorb an additional quantity of normal straight chainhydrocarbons in the adsorption step. It has been found that maintainingthe adsorption vessel at a pressure of between 10 to 50 p.s.i.g. duringthe adsorption step affords good results in terms of rapid adsorption ofthe adsorbable components of the feed stream by the selective adsorbent.

The charge stock is introduced into the adsorption vessel at a selectedrate and the feed is continued until the selective adsorbent is loadedwith normal straight chain components of the feed. Introduction of feedis preferably continued beyond the point at which the straight chaincomponents of the feed begin to breakthrough into the adsorptioneffluent (non-adsorbed portion of the feed). Introduction of the feedinto the adsorption vessel is preferably terminated when there is anormal paraffin overcharge of between about 0.5 and 15 weight percent.

Normal paraffin overcharge is defined as the amount of n-parains in thefeed stock to the adsorption vessel which is charged during theadsorption step in excess of the total amount on a weight basis ofrecovered normal parains during desorption and the normal parainsrecovered in the depressuring and purge effluent streams expressed as apercentage of normal parains charged. The utilization of the selectiveadsorbent at maximum eiciency is a material factor in the process of thepresent invention because it compensates for the less than completedesorption of the adsorbed straight chain hydrocarbons in the subsequentdesorption step. The overcharge range of 0.5- weight percent is suitablefor excellent sieve utilization in a short processing time.

After termination of the adsorption step the adsorption vessel isdepressured in a depressuring step to a lower pressure than theadsorption pressure. This depressuring step is required to remove someof the surface adsorbed non-straight chain hydrocarbons from theselective adsorbent and to also begin to remove from the adsorptionvessel, particularly from the Void spaces between the selectiveadsorbent some of the unadsorbed portion of the charge stock whileminimizing loss of the adsorbed straight chain hydrocarbons from thesieve pores.

The depressuring step is terminated when the adsorption pressure isdecreased to about atmospheric pressure, and advantageously in the rangeof 0-10 p.s.i.g. The depressuring step is carried out at substantiallythe same temperature as was used in the adsorption step.

Following termination of the depressuring step, a purge step is begunusing as the purge medium a vaporized slip-stream of the same materialsubsequently used as the desorbing medium. The purge step is carried outat substantially the same temperature as the adsorption and depressuringsteps, and at a reduced pressure attained in the depressuring step. Inthis purge step a stream of the vaporized desorption medium isintroduced into the adsorption vessel in a direction countercurrent tothe flow of the charge stock thereto. The purge medium removes theremaining portion of the charge stock from the adsorption vessel and thesurface adsorbed nonstraight chain components from the selectiveadsorbent. In the purge step it is necessary to maintain the purgemedium in the vapor state for eiicient operation and the flow ratethereof at a value between 50 and 1000 vapor hourly space velocity(volume of vapor per hour per volume of adsorbent) and the purge volumeat a value between 0.1 and 10 volumes and wherein the ratio of the purgerate to the purge volume is between 40 and 7000 to minimize removal ofthe pore adsorbed straight chain components of the feed stream and tomaximize removal of surface-adsorbed and the bed-entrapped contaminatingcomponents. The term vapor hourly space velocity refers to the purgemedium charge rate expressed as vapor volumes (at purge conditions) perhour per volume of adsorbent. The term purge volume refers to the amountof purge medium in the purge effluent stream per cycle and is equivalentto one vapor volume displacement (at purge conditions) of total volumeoccupied by the sieve bed. Most efficient operations are conducted usinga purge velocity of 170 to 680 vapor hourly space velocity and a purgemedium volume of 0.2 to 6.0 and a purge gas rate to purge volume ratoofat least 50/1, when it is desired to attain exceptionally highn-paraflin product purity. The effluent from the purge step comprisingpurge medium, unadsorbed charge stock and surface adsorbed components ofthe charge stock together 4with some adsorbed n-parans removed from thesieve pores by the purge medium is returned to the fresh feed line as asupplemental charge to the adsorption vessel. Routing of the purgeeffluent in this manner permits readsorption by the sieve of the normalstraight chain hydrocarbon components of the feed that had been removedtherefrom in the purge step. In addition the normal praflins in thepurge stream effluent are recovered in the process.

After completion of the purge step, the vessel is repressured to thedesorption pressure which is advantageously about 11-75 p.s.i.g., andpreferably about 1-25 p.s.i.g. above the highest pressure in the sievevessel during the adsorption step. This repressuring step is necessaryto permit more rapid desorption of the pore adsorbed straight chaincomponents from the adsorbent and to facilitate removal of thesecomponents from the sieve by the desorbing medium in the desorptionstep. The desorption pressure is attained by discontinuing the flow ofthe purge effluent stream to the purge accumulator via line 28, whilecontinuing the flow of purge medium into the adsorption vessel. The rateof flow of the desorbing medium into the adsorption vessel is about0.25-3 liquid hourly space velocity (LHSV) to remove the pore adsorbedstraight chain hydrocarbons from the sieve. The desorption efiiuentcomprising a mixture of desorbed straight chain hydrocarbons anddesorbing medium is recovered from the adsorption vessel and thentreated to separately recover the desorbing medium and the desorbedstraight chain hydrocarbons.

In the desorbing step, the desorbing medium employed is essentially ofthe same composition as the purge medium. Use of the same hydrocarboncomposition as the purge and desorption media avoids the problem ofproduct contamination with other hydrocarbons while simplifying theprocessing requirements. Choice of a suitable desorption medium for usein the practice of the present invention is largely dependent on thecomposition of the fresh feed, desorbent avails thereof and desired endproduct carbon number distribution. In general it has been found thatmost advantageous results are obtained when the desorption mediummixture is composed of a major amount, from 60 to about 90% by volume ofthe mixture of straight chain hydrocarbons having an average of about 1to 3 carbon atoms less than the lightest straight chain hydrocarbon inthe fresh feed charge to the adsorption vessel and a minor amount from10 to about 40% by volume, of at least one light straight chainhydrocarbon or a mixture of straight chain hydrocarbons having anaverage of 5 to 7 carbon atoms less than the lightest straight chainhydrocarbon in the fresh feed charge to the adsorption vessel.Maintaining a carbon number spread of about l to 3 for the heavydesorption medium component and 5 to 7 for the light desorption mediumcomponent between the purge-desorption media and the fresh feed chargelightest component permits effective and rapid desorption times in theprocess of the present invention in addition to affording ease ofseparation of the desorbing medium from the desired product stream byfractionation. An additional and unexpected advantage is that less ofthe adsorbed straight chain hydrocarbon components of the feed areremoved from the sieve pores during the purge step. Advantageously inthe treatment of C13-C18 charge stocks, a purge-desorption mediummixture comprising about by volume of normal undecane and 20% by volumeof normal heptane has been found to be satisfactory. In processingheavier stocks, e.g., C14-C20 containing stocks, a desorption mediummixture composed of 70 to 90% by volume of C11-C13 straight chainhydrocarbons and l0 to 30% by volume of Cri-C9 straight chainhydrocarbons has been found to give excellent results.

In the prior processes, the desorption step is generally the limitingfactor in overall process time because of the amount of time required toeffect removal of the adsorbed components from the sieve pores. Thepresent process provides an improvement over prior processes by thecombination of (a) using a desorbing medium admixture in the vapor stateand comprising in itself an adsorbable normal parain hydrocarbonmixture, (b) a desorption medium mixture space velocity of 0.25 to 3LHSV, and (c) additionally in terminating the desorption step when about10*30% by weight of the pore adsorbed straight chain hydrocarboncomponents remain in the sieve pores. Such a desorbing combination willmaterially improve the desorption of the adsorbed straight chaincomponents from the sieve pores. It has been found that at a desorptiontemperature of about 655 F. employing a 20% by volume n-heptane, 80% byvolume n-undecane desorption medium mixture in the desorption ofn-Cla-Cgo components from the adsorbent pores at a desorption mediumspace velocity in the range of about 2.0 LHSV, volumes liquiddesorbent/hour/volume adsorbent, the adsorbed components can be removedto the extent of 8090% in from about 16 to 22 minutes; at a 0.6 LHSV infrom about 65 to 89 minutes.

In the desorption step of the present invention, the tiow of desorbingmedium admixture into the adsorption zone is countercurrent to the freshfeed charge which preferably is upow. By operating in this manner, thelighter straight chain hydrocarbon components of the charge adsorbed inthe pores of the adsorbent during the adsorption step are iirstdesorbed, and, in turn, they assist the desorbing medium admixture indesorbing the adsorbed heavier straight chain hydrocarbon componentsnearer to the desorption outlet end of the vessel. Termination of thedesorption cycle short of essentially complete removal of adsorbedstraight chain hydrocarbons from the sieve pores permits the time ofdesorption to be materially decreased, i.e. in the order of 25-S0%.Moreover, the throughput of the charge can be materially increased withthe result that more charge stock can be treated per operating day andmore product can be obtained.

At the termination of the desorption step, the adsorption vessel isdepressured to the adsorption pressure and the cyclic operation isrepeated.

While the above detailed description of the process of the presentinvention has referred to a single vessel operation for simplicity, itis within the purview of the invention to produce same on a multi-vesselbasis, wherein one or more separate vessels are used in each of the mainprocess steps, i.e. adsorption, purge and desorption while another setof vessels are on a regeneration cycle. Periodic regeneration of theselective adsorbent is needed to restore the activity thereof after usein the process for an extended processing period. Suitable regenerationtechniques known in the art such as, for example, the process `disclosedin the Carter et al. U.S. Pat. 2,908,639 can be used.

The process of the present invention is essentially a timed cyclicprocess. It has been found that in cases where a relatively longdesorption time is required satisfactory results have been achieved ifthe adsorption step is accomplished in one-third of the total processingtime, the remaining two-thirds being taken up by the balance of theprocessing steps, eg. depressure, purge, repressure, desorption anddepressure. In general in processing gas oil type charge stocks torecover the straight chain hydrocarbon components thereof it has beenfound that the following time sequence is advantageous: adsorption, 18.5minutes; depressure, 0.5 minute; purge 1.0 minute; repressure, 0.5minute; desorption, 35.0 minutes; a total cycle time of 55.5 minutes.

Under certain circumstances wherein the feed stock properties, carbonnumber distribution of straight chain hydrocarbon product, desorbingmedium employed, etc., result in very short desorption times, it is moreadvantageous to accomplish the adsorption step in about onehalf of thetotal processing time with the remaining time being taken up with thedepressure, purge, repressure, desorption and depressure steps.

With reference to the accompanying drawing, in the adsorption step, thevalves in lines Z6, 36, 41, 43 and 44 are in the closed position. At thetermination of the adsorption step the valve in line 41 opens andpermits the desorbing medium mixture maintained in the lines 40 and 41under pressure and at elevated temperature, to be by-passed around theadsorption vessel. At the same time the valve in line 26 is opened todecrease the pressure in the adsorption vessel 14A (on the purge cycle).Then the valve in line 44 is opened to permit passage of a stream ofdesorbing medium mixture into vessel 14A for the purging step. At thecompletion of the purging step, the vessel is repressured by the flow ofthe stream of desorbing medium mixture into the vessel until thedesorption pressure is reached. The valves in lines 41 and 44 are thenclosed and the valves in lines 43 and 36 are opened substantiallysimultaneously with the closing of valves in lines 41 and 44. At theconclusion of the desorption step the valves in lines 43 and 36 areclosed. Operating with this valve switching sequence permits the yieldof high purity normal paraiiins to be increased without damaging thesieve bed by pressure variations during this portion of the cycle. Thisembodiment further permits use of loW purge volume displacements duringthe purge step and minimizes the loss of adsorbed normal parafiins fromthe sieve pores during the purge cycle.

In carrying out the process of the present invention under circumstancesin which a relatively long desorption time is required, it has beenfound advantageous to employ a three sieve case system wherein one sievecase is on the adsorption cycle and the remaining two cases are on thedesorption cycle (i.e. includes the depressure, purge, repressure anddesorption steps). Operating with two cases on the desorption cyclepermits a lower desorbing medium mixture space velocity to be employedsince the available desorption time is lengthened for a given totalcycle time. The beneficial results obtained by operation in this mannerinclude increased sieve utilization at a given desorption rate or lowerdesorption medium requirements at the same desorption rate. It isnecessary to carry out desorption of the two sieve cases on thedesorption cycle in parallel to prevent readsorption of the desorbednormal parains at the inlet of the second sieve case. Series desorptionin the sieveecases is to be avoided for this reason.

Following is a description by way of example of a method of carrying outthe process of the present invention.

EXAMPLE I A hydrotreated gas oil fraction having a boiling point rangeof 500 to 574 F. and containing 22.1% by weight of C13-C18 straightchain hydrocarbons is charged at a temperature of 655 F. and a pressureof 12 p.s.i.g. 'and a feed rate of 2816 gm./hr. to the lower end of anadsorption Vessel measuring 43 inches by 3 inches in diameter, having aninternal volume of about 0.2 cubic feet and containing about 8.7 poundsof J/lf; inch extruded molecular sieve selective adsorbent, sold underthe trade name Linde Molecular Sieve 'Iype 5A-45. There is recoveredfrom the other end of the vessel an adsorption eiiiuent streamcomprising 71.6% by weight C13-C18 nonstraight chain hydrocarbons, 2.0%by weight of C13-C18 straight chain hydrocarbons, and 26.4% by Weight ofthe desorbing medium mixture. The recovered adsorption eflluent isfractionated and there is separately recovered C13-C18 non-straightchain hydrocarbons in la yield of 80.1% by weight, basis fresh feed. Inthe adsorption vessel the selective adsorbent adsorbed the straightchain hydrocarbon components of the feed to the extent that after about16 minutes on the adsorption cycle the adsorbent is saturated with thestraight chain hydrocarbon components. The flow of feed is continued tothe adsorption zone until a total time of 18.5 minutes elapsed at whichtime there is a 10% overcharge of straight chain hydrocarbons therein.The feed into the adsorption vessel is then discontinued and the vesseldepressed to about 1.0 p.s.i.g. in 0.5 minute. After attaining thereduced purge pressure, a purge stream of the desorbing medium mixturein the vapor state comprising by volume of n-undecane, and 10% by volumeof n-heptane, is passed into the adsorption vessel at a rate of 362vapor hourly space velocity (V0/hr./VA) and countercurrent to thedirection of the' feed thereto. The flow of the purge medium iscontinued for 1.0 minute at which time 5.0 purge volumes had been used.The ratio of the purge medium vapor hourly space velocity to the purgevolume is about 72/ 1. The purge efliuent comprising 11.5% by weight ofsurface adsorbed materials, 72.9% by weight of purge medium and 15.6% byweight of pore adsorbed C13-C18 straight chain hydrocarbons, is removedat a 7130 gm./ hr. rate, then passed through a cooler-accumulator toreduce the temperature and pressure to about 90 F. and 1.0 p.s.i.g.

After a purge period of about 1.0 minute, the flow of purge effluentfrom the adsorption vessel is discontinued.

The desorbing medium mixture, having the same composition as the purgemedium described above, is passed in the vapor state at 655 F. into theadsorption vessel in the same direction as the purge medium at a rate of1.5 liquid hourly space velocity (V/hr./VA) to repressure the vessel tothe desorption pressure of about p.s.i.g. The repressuring operation iscompleted in about 0.5 minute. The flow of desorbing medium mixture iscontinued for the remainder of 35.0 minutes of the desorption period.There is recovered a desorption effluent which on subsequent separationyields the following fractions: C13-C18 straight chain hydrocarbons,19.9% by weight yield, basis fresh feed, 93.1% by weight desorbingmedium mixture, basis total desorbing medium feed (including purge).

The flow of the desorbing medium mixture to the adsorption vessel isdiscontinued when 90% of the pore adsorbed straight chain hydrocarbonsare removed from the sieve pores. The C13-C13 straight chain hydrocarbonproduct has a purity of 97.8% by Weight and is obtained in a yield of90.0%, by weight, basis fresh feed content of straight chainhydrocarbons. The sieve utilization value is 1.134 pound of C13-C18straight chain hydrocarbon product per day per pound of molecular sieve,a 6.4% increase over the utilization value of Comparative Example Abelow.

EXAMPLE II The procedure of Example I above is repeated except that thedesorbing medium mixture composition is 80% by weight of n-undecane, 20%n-heptane. The straight chain hydrocarbon overcharge in the adsorptionperiod is 10%, and the percentage desorption of straight chainhydrocarbons in the desorption period is 90%. The Sieve utilizationvalue is 1.152, a 8.1% increase over the utilization value ofComparative Example A below. The C13-C18 product purity is 97.8%.

EXAMPLE III The procedure of Example I is repeated except that thedesorbing medium mixture composition is 70% by weight of n-undecane,n-heptane. The percentage overcharge in the adsorption period is 10%,and the percentage desorption in the desorption period is 90%. The sieveutilization value is 1.132, a 6.2% increase over the utilization valueof Comparative Example A below. The C13-C18 product purity is 97.8%.

COMPARATIVE EXAMPLE A The procedure of Example I is repeated except thatthe desorbing medium composition is 100% n-undecane. The percentageovercharge and the percentage desorption are 10% and 90%, respectively.The sieve utilization value is 1.066 and the product purity is 97.7%

The above Examples I-III demonstrate that by carrying out the process ofthe present invention in the manner herein described one is able toattain an increased sieve utilization value which results in morestraight chain hydrocarbon product being produced per day than by priorprocesses using conventional desorbing medium.

It has been found that using more than about by volume of the lightstraight chain hydrocarbon in the heavy straight chain hydrocarbondesorbing medium does not produce any additional beneficial effect interms of reduced loss of the sieve adsorbed straight chain hydrocarboncomponents of the feed without a corresponding decrease in processefficiency in terms of slower desorption rate. The result is a net lossin product yield.

The use of less than about 10% of the light straight chain hydrocarbonsin the desorbing medium should be avoided because the loss of the poreadsorbed straight Cil chain hydrocarbon components of the feed duringthe purge period is correspondingly higher.

It is surprising that the addition of minor amounts of a lower boilingstraight chain hydrocarbon to a high boiling straight chain hydrocarbondesorbing medium results in an unpredictable improvement in theefficiency of a straight chain hydrocarbon separation process. One wouldexpect that such an addition would be detrimental since the presence oflower boiling straight chain hydrocarbons decreases the desorption rate,i.e. less straight chain hydrocarbons are removed from the sieve poresper unit of desorbing medium.

In the process of the present invention, it has been found that thedecrease in desorption rate due to the presence of minor amounts oflower boiling straight chain hydrocarbons in the desorbing medium ismore than offset by the advantages resulting during the purge periodwherein a reduced amount of adsorbed straight chain hydrocarbonscomponents of the feed is removed from the molecular sieve during thepurge step of the process. The net result is improved sieve utilization,i.e., more straight chain hydrocarbon product per day per pound ofmolecular sieve.

An additional advantage accrues from the addition of a minor amount oflower boiling straight chain hydrocarbons to the higher boiling straightchain hydrocarbon desorbing medium. `In the process of the presentinvention the purge stream effluent is passed from the adsorption vesselthrough an accumulator and blended with the fresh feed to the adsorptionvessel during the adsorption step. The presence of the lower boilingstraight chain hydrocarbons in the desorbing medium which also is usedas the purge medium during the purge period aids in reducing the dewpoint of the fresh feed which is subjected to vaporization before beingintroduced into the adsorption vessel during the adsorption step. Thisreduction in the fresh feed dew point permits the feed stream to bevaporized at a lower temperature and avoids undesirable ther mal sidereactions such as cracking and polymerization usually associated withheating of heavy petroleum fractions at relatively high temperatures,i.e. in the order of 650 F .and upwards.

Obviously, many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof and therefore only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. In a cyclic vapor phase process for the separation of a petroleumfraction using a molecular sieve selective adsorbent, comprising anadsorption step, a purge step and a desorption step, the improvementwhich comprises carrying out the purge and the desorption steps with adesorbing medium consisting essentially of a straight chain hydrocarbonmixture composed of a major amount of at least one heavy straight chainhydrocarbon having a molecular weight of 1 to 3 carbon atoms less thanthe molecular weight of the lighest straight chain hydrocarbon componentof the petroleum fraction being separated and a minor amount of at leastone light straight chain hydrocarbon having a molecular weight of from 5to 7 carbon atoms less than the molecular weight of the lighest straightchain hydrocarbon component of the petroleum fraction being separated.

2. The process of claim 1 wherein the petroleum fraction is a ygas oilcontaining C13-C18 straight chain and nonstraight chain hydrocarbons.

3. The process of claim 1 wherein the petroleum fraction is a gas oilcontaining C13-C20 straight chain and non-straight chain hydrocarbons.

4. The process of claim 1 wherein the straight chain hydrocarbon mixtureused as the desorbing medium contains from 60 to 90% by volume of saidheavy straight chain hydrocarbon and 10 to 40% by volume of said lightstraight chain hydrocarbon.

5. The process of claim 1 wherein the straight chain hydrocarbon mixtureused as the desorbing medium contains from 70 to 90% by volume of saidheavy straight chain hydrocarbon and 10 to 30% by volume of said lightstraight chain hydrocarbon.

6. The process of claim 1 wherein the straight chain hydrocarbon mixtureused as the desorbing medium contains from 75 to 85 by volume of11undecane and 15 to 25% by volume of n-heptane.

7. The process for the separation of C13-C20 straight chain hydrocarbonsfrom a petroleum fraction which comprises in an adsorption stepcontacting said petroleum fraction in the vapor phase at an elevatedtemperature and pressure and with a A. type molecular sieve selectiveadsorbent to adsorb the straight chain hydrocarbons, in a purge step ata lower pressure countercurrently contacting said molecular sieve with avaporized desorbing medium consisting essentially of a straight chainhydrocarbon mixture containing a major portion of at least one heavystraight chain hydrocarbon having a molecular Weight of l to 3 carbonatoms less than the lightest straight chain hydrocarbon component of thepetroleum fraction and a minor portion of at least one light straightchain hydrocarbon having a molecular Weight of 5 to 7 carbons less thanthe lightest straight chain hydrocarbon component of the petroleumfraction, at a purge medium rate of between 50 and 1000 vapor hourlyspace Ivelocity and a purge medium volume in the range of 0.1 to volumesof vapor per unit bed Nolume and wherein the ratio of the purge mediumrate to the purge medium volume is between 1 and 7000/ l, and in adesorption step at a higher pressure than the adsorption pressure,removing the adsorbed straight chain hydrocarbons from the molecultarsieve with a Vaporized desorbing medium mixture having the samecomposition as the desorbing medium employed in the purge step.

8. The process of claim 7 wherein the desorbing medium contains from to90% by volume of said heavy straight chain hydrocarbon and 10 to 40% byvolume of said light straight chain hydrocarbon.

9. The process of claim 7 wherein the desorbing medium contains from to85% by volume of n-undecane and from 15 to 25% by volume of n-heptaneReferences Cited UNITED STATES PATENTS 2,891,902 6/1959 Hess et al208-65 3,370,002l 2/1968 Cottle 208-310 3,373,103 3/1968 Cooper et al208-310 3,392,113 7/1968 De Rosset 208-310 3,395,097 7/1968 Senn 208-310HERBERT LEVINE, Primary Examiner U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO-3,539.501 Dated Nnvpmhm 10, 1970 Inventor(s4) Rlchard w' Stokeld, JT.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column l, line 42, "hydrocarbon" should read hydrocarbons line 53, "z"should read Column 2, line 39, "hydrocarbon" should read hydrocarbonsline 40, "straight", second occurrence, should read still linl 46,"acomplished" should read accomplished line 5l, "acrylic" should readacylc Column 3, line 66, "pressuring" should read depressuring line 68,"line" should read lines Column 6, line 25, after "mixture" insert of atleast one heavy straight chain hydrocarbon or a mixture Column 8, line60, "depressed" should read --v depressured Signed and sealed this 9thday of March 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents FORM IPO-1050 (1o-69) USCOMM.DC 60375. e ussovznunzn'r "umu: omc: nu o-:c

